High-speed data communication systems commonly use multiplexing transmitters and demultiplexing receivers. In such a system, as illustrated in FIG. 1, transmit data arrives on a set of parallel lines 113 and is multiplexed onto the transmission line 114. The multiplexer converts the parallel signal at the reference clock rate on lines 113 into a serial signal at the bit clock rate on line 114. At the other end of the link, the serial signal arrives on input line 115 and is demultiplexed onto parallel outputs 116. In such a system, the bit clock that sequences multiplexer 102 and demultiplexer 103 has a frequency that is a multiple of the reference clock 111 used to clock parallel input 113 and parallel output 116. In the example of FIG. 1 where the multiplexing rate is 4:1, the bit clock would be at four times the frequency of the reference clock. In actual systems, a ratio of 10:1 or 20:1 is typical.
Phase-locked loop clock multipliers have been used to multiply the frequency of the reference clock to generate the bit clock. In FIG. 1 clock 111 is input to clock multiplier 101 which multiplies the clock frequency to generate bit clock 112. Bit clock 112 is then used by multiplexer 102 to multiplex parallel input 113 onto the input of driver 104 which drives the multiplexed data onto output line 114. The bit clock is also used by demultiplexer 105 to separate the multiplexed input stream on input line 115 onto parallel outputs 116.
FIG. 2 illustrates a prior art phase-locked loop clock multiplier. The bit clock, bclk is generated by voltage-controlled oscillator 121. This clock is then divided down to the reference clock rate by a divide-by-N counter 122. The divided clock, dclk, is then compared to the input reference clock, rclk, by phase comparator 123. The phase comparator signals the phase difference between rclk and dclk to the charge pump and loop filter 124 which adjusts the control voltage of the VCO to bring rclk and dclk into phase. Further details of phase-locked loops are described in Dally and Poulton, Digital Systems Engineering, Cambridge, 1998, pp 441-447.
An alternative prior art multiplexing data communication system that uses a multi-phase clock rather than a clock multiplier is illustrated in FIG. 3. In the figure a four-phase clock, p1-p4, is used to multiplex parallel lines 113 onto output line 114 and to demultiplex serial input 115 onto parallel lines 116. The four-phase clock is generated by a delay-locked loop (DLL) comprising tapped delay line 131, phase comparator 123, and charge pump 124. The tapped delay line is itself composed of four delay elements 132-135. Phase comparator 123 compares the output of the delay line, p4, with the reference clock and signals the charge pump to adjust the control voltage, vctrl, of the delay line to bring p4 and rclk into phase. When the loop has converged, vctrl is set at a value that causes delay line 131 to have a delay of exactly one reference clock cycle. To the extent that delay elements 132-135 are matched, the four phases are equally spaced with one bit-time of delay between each phase. In the multiplexer, the rising edge of each phase sequences the corresponding bit onto the line, and in the demultiplexer the rising edge of each phase samples the value on the line onto the corresponding parallel output. Further details of delay-locked loops are described in Dally and Poulton, Digital Systems Engineering, Cambridge, 1998, pp 428-441, and details of multiplexing data communication systems using DLLs are described in pp. 537-540 and 547-548 of the same reference.
Prior art data communication systems based on PLL clock multipliers and multi-phase DLLs have large amounts of jitter due to the method used to generate timing signals. Phase-locked loop based clock multipliers have large amounts of jitter because the phase error at the end of each cycle accumulates until the control loop can respond. As described in Kim, Weigant, and Gray, “PLL/DLL System Noise Analysis for Low Jitter Clock Synthesizer Design,” ISCAS 1994, pp. 31-38, this error accumulation multiplies the jitter of the basic delay elements by a factor that is inversely proportional to the loop bandwidth. For typical phase-locked loops, the jitter is multiplied by a factor of at least 10.
Communication systems based on multi-phase delay-locked loops do not accumulate jitter from cycle-to-cycle like PLL clock multipliers. However, they do introduce jitter due to cumulative phase mismatches. Due to device mismatches in the delay elements, there is a variation in the delay of each stage of the delay line. These phase mismatches accumulate over the length of the delay line leading to large jitter values, particularly when the multiplexing rate is high.